The present invention is directed to printing processes employing microwave image drying techniques. More specifically, the present invention is directed to printing processes which comprise applying in imagewise fashion to a substrate an ink composition which comprises an aqueous liquid vehicle, a colorant, and an ionic compound at least partially ionizable in the liquid vehicle, said ink composition having a conductivity of at least about 10 milliSiemens per centimeter, and subsequently exposing the substrate to microwave radiation, thereby drying the images on the substrate. Any printing process suitable for use with aqueous-based inks can be employed. A preferred embodiment of the present invention is directed to ink jet printing processes using specific ink compositions and employing microwave drying of the printed images. One specific embodiment of the present invention is directed to a thermal ink jet printing process which comprises (1) incorporating into a thermal ink jet printing apparatus an ink composition which comprises an aqueous liquid vehicle, a colorant, and an ionic compound at least partially ionizable in the liquid vehicle, said ink composition having a conductivity of at least about 10 milliSiemens per centimeter; (2) heating the ink in an imagewise pattern to cause bubbles to form therein, thereby causing droplets of the ink to be ejected in an imagewise pattern onto a substrate, thereby generating images on the substrate; and (3) exposing the substrate to microwave radiation, thereby drying the images on the substrate.
Ink jet printing systems generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging, or deflection, they are much simpler than the continuous stream type. There are two types of drop-on-demand ink jet systems. One type of drop-on-demand system has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the system's ability to produce high quality prints or images. Drop-on-demand systems which use piezoelectric devices to expel the droplets also suffer the disadvantage of a slow printing speed.
The second type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets and allows very close spacing of nozzles. The major components of this type of drop-on-demand system are an ink-filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle causing the ink in the immediate vicinity to evaporate almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. When the hydrodynamic motion of the ink stops, the process is ready to start all over again. With the introduction of a droplet ejection system based upon thermally generated bubbles, commonly referred to as the "bubble jet" system, the drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous stream counterparts, and yet have substantially the same high speed printing capability.
The operating sequence of the bubble jet system begins with a current pulse through the resistive layer in the ink filled channel, the resistive layer being in close proximity to the orifice or nozzle for that channel. Heat is transferred from the resistor to the ink. The ink becomes superheated far above its normal boiling point, and for water based ink, finally reaches the critical temperature for bubble formation or nucleation of around 280.degree. C. Once nucleated, the bubble or water vapor thermally isolates the ink from the heater and no further heat can be applied to the ink. This bubble expands until all the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor, which removes heat due to heat of vaporization. The expansion of the bubble forces a droplet of ink out of the nozzle, and once the excess heat is removed, the bubble collapses on the resistor. At this point, the resistor is no longer being heated because the current pulse has passed and, concurrently with the bubble collapse, the droplet is propelled at a high rate of speed in a direction towards a recording medium. The resistive layer encounters a severe cavitational force by the collapse of the bubble, which tends to erode it. Subsequently, the ink channel refills by capillary action. This entire bubble formation and collapse sequence occurs in about 10 microseconds. The channel can be refired after 100 to 500 microseconds minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to become somewhat dampened. Thermal ink jet processes are well known and are described, for example, in U.S. Pat. Nos. 4,601,777, 4,251,824, 4,410,899, 4,412,224, and 4,532,530, the disclosures of each of which are totally incorporated herein by reference.
Inks for ink jet printing processes are known. For example, U.S. Pat. No. 3,994,736 (Hertz et al.) discloses a pigment-free high intensity light fast ink suitable for ink jet printing which comprises a polar solvent liquid base and a premetallized azo dye. The ink can also contain an inorganic ionizable salt to impart electrical conductivity. Examples of salts include those with a chloride or nitrate anion and an alkali metal cation or ammonium cation In addition, U.S. Pat. No. 4,142,905 (Cooke) discloses a waterless ink for non-impact jet printers which comprises an ethylene glycol vehicle, an inorganic salt such as sodium nitrite, sodium nitrate, or sodium chloride dissolved in the glycol to provide a required level of electrical conductivity, and a dye. Further, U.S. Pat. No. 4,070,322 (Hwang et al.) discloses an ink composition for jet printing onto non-absorbent surfaces based on a ketone solvent having a soluble ionizable salt and a dyestuff. In addition, the ink can contain water in an amount of up to 10 percent by weight, a lower alcohol in an amount of up to 8 percent by weight, a pyrrolidone in an amount of up to 4 percent by weight, and a vinyl acetate copolymer resinous binder in an amount of up to 6 percent by weight. The ink has a viscosity of 1.5 to 10 centipoise, a resistance of less than 2,000 ohm-cm, a surface tension of less than 28 dynes per centimeter, a velocity of sound within the range of 1,200 to 1,700 meters per second, and an upper limit of 5 microns insolubles. Examples of soluble ionizable salts include the alkali metal and alkaline earth metal halides, ammonium nitrate, and alkali metal thiocyanates, alkali metal alkylates. Additionally, U.S. Pat. No. 4,256,493 (Yokoyama et al.) discloses a jet ink composition which comprises an aqueous jet ink containing a water-soluble dye, a wetting agent, and water as main components and, incorporated therein, a water-soluble ultraviolet absorbing agent as well as a metal salt, when necessary. The metal salt component is capable of improving the light-resistance of the ink.
U.S. Pat. No. 4,839,142 (Charm), the disclosure of which is totally incorporated herein by reference, discloses a high temperature, short time heating system and method for the pasteurization and/or sterilization of heat sensitive biological fluids which comprises adding a dielectric enhancing additive to the biological fluid, subjecting the biological fluid to microwave energy to heat rapidly the biological fluid for a short time period to a pasteurizing or sterilization temperature, cooling the biological fluid, optionally removing the dielectric enhancing additive, and recovering an aseptic biological fluid. Examples of dielectric enhancing additives include inorganic metal or ionic salts, such as alkali or alkaline earth salts, such as sodium chloride.
U.S. Pat. No. 4,327,174 (von Meer), the disclosure of which is totally incorporated herein by reference, discloses a carrier material for photographic purposes which is formed of paper coated with a synthetic resin. The paper contains a water soluble inorganic salt that enhances its drying by microwaves. Examples of soluble inorganic salts include KCl, NaCl, Na.sub.2 SO.sub.4, NaBr, KBr, MgCl.sub.2, MgSO.sub.4, CaCl.sub.2, ZnCl.sub.2, LiCl, and K.sub.2 SO.sub.4.
Copending application U.S. Ser. No. 07/829,863, filed Feb. 3, 1992 the disclosure of which is totally incorporated herein by reference, discloses printing processes which comprise incorporating into an ink jet printing apparatus an ink composition which comprises an aqueous liquid vehicle, a colorant, and a zwitterionic or potentially zwitterionic component such as 4-aminobutyric acid, 6-aminocaproic acid, L-proline, and the like as well as mixtures thereof; (2) causing droplets of the ink to be ejected in an imagewise pattern onto a substrate, thereby generating images on the substrate; and (3) exposing the substrate to microwave radiation, thereby drying the images on the substrate.
Although known compositions and processes are suitable for their intended purposes, a need remains for ink jet printing processes with rapid output times. In addition, there is a need for ink jet printing processes employing microwave drying of the images. Further, there is a need for thermal ink jet printing processes that enable output speeds of at least 10 prints per minute, as well as slower output speeds. There is also a need for ink jet printing processes employing microwave drying wherein the microwave drying apparatus has reduced power requirements. Further, a need exists for thermal ink jet printing processes with fast image drying times and with reduced paper cockle. In addition, there is a need for printing processes for which aqueous-based inks are suitable and which employ microwave drying of the printed images. There is also a need for thermal ink jet printing processes employing microwave drying of the images which enable good print quality and minimal showthrough and strikethrough.